1. Field of the Invention
The present invention relates to a developing apparatus which may be incorporated in an image forming machine such as an electrophotographic printer or electrostatic-recording printer. More specifically, the present invention relates to a developing apparatus of the type which includes two or more developing rollers.
2. Description of the Related Art
Recently, an electrophotographic printer is increasingly used which has a developing apparatus equipped with two or more developing rollers in order to meet a demand for higher printing speed. Such a developing apparatus effectively supplies a sufficient amount of developer composition to an electrostatic latent image carrier such as a photosensitive drum for development of the latent images, thereby enabling thick printing.
A developing apparatus of the above-described type is exemplified in FIG. 9. As shown in this figure, a photosensitive drum 10 rotates clockwise (the arrow P direction in FIG. 9). The drum 10 faces a first or lower developing roller 20 and a second or upper developing roller 30. Each of the developing rollers 20, 30 includes a magnetic roll body 21 (31) which is a fixed cylinder with N-poles and S-poles alternately arranged circumferential, and a rotary sleeve 22 (32) of e.g. stainless steel rotatably fitted on the roll body. The rotary sleeve 22 (32) rotates clockwise (the arrow P direction), preferably at a higher peripheral speed than the photosensitive drum 10.
Both of the developing rollers 20, 30 are disposed in a container 40 containing a suitable amount of developer composition S which is a mixture of toner with small magnetic particles as carrier. The toner is replenished as consumed, from a non-illustrated toner supplier, and thus the developer composition is maintained at a predetermined toner concentration. Indicated by reference numeral 41 is an agitator for helping the developer composition circulate within the container 40.
The developer composition S is electrostatically charged by mutual friction between the toner particles and the carrier particles while being agitated by the agitator 41. When moved onto the first developing roller 20 by the agitator 41, the developer composition is attracted on the rotary sleeve 22 by the magnetism of the magnetic roll body 21 and then supplied onto the photosensitive drum 10 as the rotary sleeve 22 rotates. Specifically, the developer composition on the rotary sleeve 22 ears up like whiskers along the magnetic flux arcuately extending from the N poles to the S poles around the magnetic roll body 21 and is transferred in this state. The ear-up state of the developer composition is appropriate for effectively depositing the developer composition on the photosensitive drum 10. At this time, the ear-up height of the developer composition is regulated by a first scraper or blade. 42 disposed under the first developing roller 20.
A remaining portion of the developer composition not passed onto the drum 10 is passed onto the second developing roller 30 and transferred thereon again in the ear-up state while coming into contact with the drum 10 for secondary deposition. A residual portion of the developer composition not passed onto the drum 10 even in the second chance for deposition moves with the rotation of the rotary sleeve 32, and is then scraped off the rotary sleeve 32 by a second scraper 43 to fall into the container 41 for recirculation.
As described hereinabove, during a single cycle of circulation within the container 40, the developer composition S comes into contact with the photosensitive drum 10 twice due to the provision of the two developing rollers 20, 30. Thus, even if the drum 10 and the developing rollers 20, 30 are rotated at a high speed for high speed printing, it is still possible to efficiently supply a sufficient amount of developer composition to the drum 10 for realizing appropriately thick printing.
However, the developing apparatus equipped with the plurality of developing rollers 20, 30 is still disadvantageous in that the carrier particles of the developer composition deteriorate in a relatively short time, and such carrier deterioration leads to a decrease of the developer composition supply to the photosensitive drum 10, consequently resulting in faint printing. This is because the circulating path for the developer composition within the container 40 is extended due to the provision of the two developing rollers 20, 30 instead of a single developing roller. In particular, the developer composition is subjected to great friction between the two rotary sleeves 22, 32 when transferred from the first developing roller 20 to the second developing roller 30. As a result, the toner powder deposits on the surfaces of the carrier particles, or the coating provided on the carrier particles is damaged, thereby resulting in improper and/or unstable electrostatic charge.
In this regard, it is possible to adjust the gray level of printing by controlling for example the charge voltage of the photosensitive drum 10. However, the early deterioration of the printing quality caused by the above-described carrier deterioration applies regardless of the gray level setting.